1. Field of the Invention
The invention relates generally to optical assemblies for lighting applications and, more particularly, to optical assemblies for solid state light sources.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is emitted from the active region and from surfaces of the LED.
In order to generate a desired output color, it is sometimes necessary to mix colors of light which are more easily produced using common semiconductor systems. Of particular interest is the generation of white light for use in everyday lighting applications. Conventional LEDs cannot generate white light from their active layers; it must be produced from a combination of other colors. For example, blue emitting LEDs have been used to generate white light by surrounding the blue LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). The surrounding phosphor material “downconverts” some of the blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light.
In another known approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes. Indeed, many other color combinations have been used to generate white light.
Because of the physical arrangement of the various source elements, multicolor sources often cast shadows with color separation and provide an output with poor color uniformity. For example, a source featuring blue and yellow sources may appear to have a blue tint when viewed head-on and a yellow tint when viewed from the side. Thus, one challenge associated with multicolor light sources is good spatial color mixing over the entire range of viewing angles.
One known approach to the problem of color mixing is to use a diffuser to scatter light from the various sources; however, a diffuser usually results in a wide beam angle. Diffusers may not be feasible where a narrow, more controllable directed beam is desired.
Another known method to improve color mixing is to reflect or bounce the light off of several surfaces before it is emitted. This has the effect of disassociating the emitted light from its initial emission angle. Uniformity typically improves with an increasing number of bounces, but each bounce has an associated loss. Many applications use intermediate diffusion mechanisms (e.g., formed diffusers and textured lenses) to mix the various colors of light. These devices are lossy and, thus, improve the color uniformity at the expense of the optical efficiency of the device.
Many modern lighting applications demand high power LEDs for increased brightness. High power LEDs can draw large currents, generating significant amounts of heat that must be managed. Many systems utilize heat sinks which must be in good thermal contact with the heat-generating light sources. Some applications rely on cooling techniques such as heat pipes which can be complicated and expensive.
Recent lighting luminaire designs have incorporated LEDs into lamp modules. There are several design challenges associated with the LED-based lamp modules including: source size, heat management, overall size of the lamp assembly, and the efficiency of the optic elements. Source size is important because the size of a 2pi emitter dictates the width of the output beam angle (i.e., etendue) using a standard aperture, such as a 2 inch (MR16) aperture, for example. Heat dissipation is a factor because, as noted above, the junction temperature of LEDs must be kept below a maximum temperature specified by the manufacturer to ensure optimal efficacy and lifetime of the LEDs. The overall size of the optical assembly is important because ANSI standards define the physical envelope into which a lamp must fit to ensure compliance with standard lighting fixtures. Lastly, the efficiency of the optic elements must be high so that the output from high-efficacy LEDs is not wasted on inefficient optics.
To address the issue of overall optical assembly size, total internal reflection (TIR) lenses have been used in lamp packages. However, TIR lenses typically exhibit poor efficiency, sometimes introducing losses of 30% or more as a tradeoff for their compact size.